During travel of a lift cage in a lift shaft different forces can act on the cage body or a cage frame holding the cage body and excite the system into vibrations. The causes for the vibrations are, in particular, unevennesses in the guide rails as well as forces produced by the slipstream about the cage, which can readily cause the cage to oscillate in a horizontal direction or about one of the two horizontal axes or about a vertical axis. In addition, lateral traction forces transmitted by the traction cables or sudden positional changes of the load during travel can be the cause of transverse vibrations.
In order to increase the travel comfort for persons using the lift and also the safety of the system, regulating systems are used which seek to counteract the forces acting on the lift cage. For example, a system is known from U.S. Pat. No. 5,896,949 which comprises several guide elements connected to the lift cage movable between two end settings, wherein vibrations arising transversely to the travel direction are detected by several sensors mounted at the cage and used for controlling several actuators arranged between the cage and the guide elements. The actuators are controlled with the help of a regulating device in such a manner that they operate in opposition to the arising forces and thus suppress the vibrations as effectively as possible.
A typical characteristic of this method for active damping of vibrations in lift cages is that the regulator output or the setting signal for the electrical actuators has to be limited, since otherwise the risk of thermal overheating exists. In the publication “Thermal Protection of Electromagnetic Actuators” of E. Cortona there is described a method in which the above-mentioned limitation of the setting signal is designed to be variable and dependent on the temperature of the actuators. It is thereby ensured that the actuators are not damaged due excessive thermal loading.
A further typical characteristic of the above-mentioned method for active vibration is that the position regulator regulating the position of the lift cage has predominantly integrating behaviour. This has the consequence that, in the case of a constant regulating deviation, the output signal of the regulator is ever greater with time. If the above-mentioned method of limiting the setting signal is applied then it can occur that the output signal of the position regulator becomes greater so long as a comparatively large regulating deviation continues. If the regulating deviation becomes smaller, there is still too long a time until the setting signal again reaches the desired value.